The second opening of My Hero Academia S3 features three rapid-fire shots of a red-haired character wearing goggles and a very smug grin (YouTube reference):

These shots are placed right after the title card, implying he's going to be a significant character in the Provisional License Exam arc, but I don't recall him actually appearing at any point. I waited until the final episode came out to ask this, just in case he showed up towards the end of the season, but as far as I can tell, he still hasn't.

I check through the lists of MHA characters on both MyAnimeList and TV Tropes, and couldn't find him on either of them.

Who is this guy? And if he didn't appear, or appeared so briefly that I forgot about him, what's the significance of his appearance in the intro?

Not sure an answer, but based on a quick search, Japanese fans on Yahoo! Chiebukuro speculated an anime-original character or Power Loader's daily costume.
– Aki TanakaSep 29 '18 at 16:43

1

Power Loader's hair is orange, not reddish-brown, so I don't think it's him. If he's an anime-original character but hasn't appeared yet, then there might not be an answer until (and unless) he does appear.
– F1KrazySep 29 '18 at 16:56